The present invention relates to an optical transmitter for converting an electrical signal into an optical signal by utilizing a semiconductor light emitting device such as a laser diode (LD) or a light emitting diode (LED) to transmit the optical signal, and particularly to a controlling technique for suppressing fluctuations in optical output characteristics of the semiconductor light emitting device due to temperature changes and the like.
Characteristics of semiconductor light emitting devices to be utilized in optical transmitters are changed corresponding to temperatures, and such temperature characteristics are different from one another, among individual semiconductor light emitting devices to be used. As such, it has been basically required for conventional optical transmitters to individually conduct adjustment of an optical output power, thereby resulting in problems of an increase in circuit size due to an increased number of adjusting parts and an increase in cost due to individual adjustment.
As a conventional technique for solving the aforementioned problems, there has been known a technique such as described in Japanese Unexamined Patent Publication No. 9-162811. This conventional technique is constituted to previously obtain temperature characteristic data concerning a laser diode and its driving circuit, store the obtained data into an optical transmitter itself, as characteristic information for control, select from the stored data characteristic information corresponding to the temperature and a targeted optical output, and control a driving current to be supplied to the laser diode from the driving circuit. In this way, it is intended to overcome, for example, a deterioration of light extinction or a delay in light emission due to the difference between the temperature characteristic of the laser diode and the temperature characteristic of the circuit for drive controlling the laser diode
In the aforementioned conventional optical transmitter, however, the temperature characteristic data as the characteristic information to be previously stored in the optical transmitter include only the driving currents (specifically, forward current and threshold current) of the laser diode, obtained corresponding to a predetermined ambient temperature and predetermined optical outputs. Therefore, it is difficult to precisely control fluctuations of the optical output power due to temperature changes.
Namely, the control of an optical output power to be performed in the conventional optical transmitter is such that the optical power to be output from the laser diode is detected by a monitoring photodiode (hereinafter called xe2x80x9cmonitoring PDxe2x80x9d), and the detected optical output power is compared with a fixed reference signal (electric cell) to control the driving current to the laser diode, so that the resultant difference obtained by the comparison is compensated for. It has been known that the monitoring PD has a photoelectric conversion efficiency which changes in a temperature dependent manner such as shown in FIG. 12. However, such a temperature characteristic of the monitoring PD has been neglected in the conventional control technique, because a variation amount of the temperature characteristic of the monitoring PD is typically smaller than a variation amount of the temperature characteristic of the laser diode. However, if the optical output power is to be controlled with a higher precision, the variation due to the temperature characteristic of the monitoring PD can never be neglected, thereby requiring a controlling technique taking account of even the temperature characteristic of the monitoring PD.
Further, in the optical output power control in the conventional optical transmitter, a so-called feedforward control is performed, in which an APC (automatic power control) using the detection result of the monitoring PD is executed only for the threshold current while a modulation current is directly controlled by the characteristic information read out from the stored data. However, it is difficult to discriminate as to whether the change of the optical output characteristic of the laser diode such as due to deterioration with time lapse is caused by the threshold current or by the modulation current. Therefore, it is difficult to perform characteristic compensation over a long time such as 25 years, by merely performing a feedback control such as the APC for the threshold current.
Moreover, in the conventional optical transmitter, to prepare the characteristic information to be stored, temperature characteristic data is obtained while changing the surrounding temperature over a required temperature range at a predetermined rate. However, to realize an optical output power control with a higher precision in such a characteristic information preparing method, it is necessary to obtain an extremely large number of temperature characteristic data. Therefore, a long period of time is required to adjust the optical transmitter, thereby causing an increase in cost due to an increase in the number of adjusting steps.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a low cost optical transmitter capable of highly precisely controlling fluctuations of optical output characteristics of semiconductor light emitting device due to temperature changes and the like.
To this end, with the present invention, an optical transmitter comprising: a semiconductor light emitting device; a driving circuit for supplying a driving current to the semiconductor light emitting device; an optical output detecting section for detecting an optical output power of the semiconductor light emitting device; and a controlling section for controlling the operation of the driving circuit based on a detection result of the optical output detecting section, further comprises: a storing section for storing temperature characteristic information including temperature characteristic data concerning the driving current to be supplied to the semiconductor light emitting device and temperature characteristic data concerning the optical output detecting section; and a temperature detecting section for detecting temperature; wherein the controlling section controls the operation of the driving circuit, based on the temperature characteristic information read out from the storing section corresponding to the temperature detected by the temperature detecting section and based on the detection result of the optical output detecting section.
In such an optical transmitter, the temperature characteristic information including the respective temperature characteristic data concerning the driving current of the semiconductor light emitting device and concerning the optical output detecting section are previously stored in the storing section, and the temperature characteristic information in the storing section is road out by the controlling section corresponding to the temperature detected by the temperature detecting section. Then, based on the read out temperature characteristic information, in the controlling section, the operation control of the driving circuit is performed while compensating for errors corresponding to temperature changes concerning the detection result of the optical output detecting section. Thereby, it becomes possible to control, with a high precision, fluctuations in temperature of the optical output power of the semiconductor light emitting device.
As a specific constitution of the optical transmitter, when the semiconductor light emitting device is a laser diode, the storing section may store therein, as the temperature characteristic information, temperature characteristic data respectively corresponding to a threshold current and a modulation current, both to be supplied to the laser diode, and temperature characteristic data of a monitoring current to be output from a light receiving element to be used in the optical output detecting section. Further, it is preferable that the controlling section of The optical transmitter feedback controls at least one of the threshold current and modulation current to be supplied from the driving circuit to the laser diode, such that the temperature characteristic data of the monitoring current read out from the storing section corresponding to the temperature detected by the temperature detecting section coincides with a monitoring current value corresponding to the optical output power detected by the optical output detecting section. When both of the threshold current and modulation current are feedback controlled, it is possible to control the operation of the driving circuit such that a value of ratio between the threshold current and modulation current in optimized at each temperature. When one of the threshold current and modulation current is feedback controlled, it is possible to feedforward control the other of the threshold current and modulation current, based an only the temperature characteristic information read out from the storing section corresponding to the temperature detected by the temperature detecting section.
According to such specific constitutions, the driving current of the laser diode which corresponds to the sum of the threshold current and modulation current is feedback controlled by the controlling section, thereby enabling to control with a high precision the optical output power of the laser diode to a required level. Note, it is also possible that the semiconductor light emitting device is a light emitting diode in the aforementioned optical transmitter, in which the optical output power of the light emitting diode can be controlled with a high precision.
Further, the temperature characteristic information to be stored in the storing section may be obtained by utilizing approximate expressions defined based on the data measured at temperatures of at least three over a working temperature range of the optical transmitter. Thereby, it becomes possible to obtain the temperature characteristic information with a higher precision concerning the working temperature range of the optical transmitter even when the number of measuring points of the temperature characteristic data is relatively less, thereby enabling to shorten the time required for the adjusting operation of the optical transmitter.
Moreover, the optical transmitter may further comprise: a driving current monitoring circuit capable of detecting an increase or decrease in the driving current to be supplied to the semiconductor light emitting device, by comparing the data corresponding to the temperature detected by the temperature detecting section, among the temperature characteristic data concerning the driving current stored in the storing section, with the driving current generated by the driving circuit. In addition, the optical transmitter may further comprise an optical output monitoring circuit capable of detecting a fluctuation in the optical output power of the semiconductor light emitting device, by comparing the data corresponding to the temperature detected by the temperature detecting section, among the temperature characteristic data concerning the optical output detecting section stored in the storing section, with the information concerning the optical output power detected by the optical output detecting section.
The above constitution makes it possible to monitor the driving state of the semiconductor light emitting device such as from the exterior, and can be utilized such as for sending an alarm when there occurs an abnormality in the driving current or in the optical output power.